A silicon single crystal is produced mainly by the Czochralski method (Hereinafter, the Czochralski method is abbreviated to the CZ method). A silicon wafer is sliced from the silicon single crystal, and is used as a substrate of a semiconductor device such as memory or CPU.
A silicon single crystal produced by the CZ method contains oxygen atoms. When a device is fabricated by using a silicon wafer sliced from the silicon single crystal, a silicon atom and oxygen atoms are bonded together to form oxide precipitates (Bulk Micro Defect; Hereinafter, it is abbreviated to BMD). It has been recognized that the BMD has the IG (Intrinsic Gettering) capability to catch contamination atoms such as heavy metal in a wafer and to improve device characteristics. Higher concentration of BMD in the bulk of a wafer provides a higher performance device.
In recent years, to control crystal defects in a silicon wafer and also to impart the IG capability sufficiently, a silicon single crystal has been produced with intentional carbon-doping or nitrogen-doping.
As for a method for doping a silicon single crystal with carbon, gas doping (See Japanese Unexamined Patent Application Publication No. H11-302099), use of high purity carbon powder (See Japanese Unexamined Patent Application Publication No. 2002-293691), use of carbon blocks (See Japanese Unexamined Patent Application Publication No. 2003-146796), and so on have been suggested. However, the above methods have problems, respectively. As for the gas doping, remelting a crystal is impossible when the crystal is dislocated. As for the use of high purity carbon powder, introduced gas scatters the high purity carbon powder at the time of melting material. As for the use of carbon blocks, carbon is hard to melt and a crystal being grown is dislocated.
To overcome these problems, Japanese Unexamined Patent Application Publication No. H11-312683 suggests a method to dope a silicon single crystal with carbon by putting a polycrystalline silicon container containing carbon powder, a silicon wafer on which a carbon film is vapor-deposited, a silicon wafer on which an organic solvent containing carbon particles is applied and baked, or polycrystalline silicon with a certain content of carbon in a crucible. Use of these methods overcomes the problems. However, every method requires a process of polycrystalline silicon, a heat treatment of a wafer, and so on. Consequently, it is not easy to prepare a carbon dopant. Furthermore, the process or the heat treatment of a wafer for preparing the carbon dopant can cause contamination by impurities.
Furthermore, Japanese Unexamined Patent Application Publication No. 2001-199794 and International Publication No. 01/79593 disclose a method for obtaining a silicon single crystal with few grown-in defects and with high IG capability by doping the crystal with carbon and nitrogen together. Doping a silicon single crystal with nitrogen is conducted commonly by mixing a wafer on the surface of which a silicon nitride film is formed into a polycrystalline material (see Japanese Unexamined Patent Application Publication No. H05-294780). However, such methods cannot overcome the above problems in carbon-doping.